Normalization of the Background Independent Open String Field Theory Action

It has been shown recently that the background independent open string field theory provides an exact description of the tachyon condensation on unstable D-branes of bosonic string theory. In this analysis the overall normalisation of the action was chosen so that it reproduces the conjectured relations involving tachyon condensation. In this paper we fix this normalisation by comparing the on-shell three tachyon amplitude computed from the background independent open string field theory with the same amplitude computed from the cubic open string field theory, which in turn agrees with the result of the first quantised theory. We find that this normalisation factor is in precise agreement with the one required for verifying the conjectured properties of the tachyon potential.Comment: LaTeX file, 8 pages, references adde

Gauge and General Coordinate Invariance in Non-Polynomial Closed String Field Theory

An appropriate field configuration in non-polynomial closed string field theory is shown to correspond to a general off-shell field configuration in low energy effective field theory. A set of string field theoretic symmetries that act on the fields in low energy effective field theory as general coordinate transformation and antisymmetric tensor gauge transformation is identified. The analysis is carried out to first order in the fields; thus the symmetry transformations in string field theory reproduce the linear and the first non-linear terms in the gauge transformations in the low energy effective field theory.Comment: 29 page

Tachyon Condensation and Brane Descent Relations in p-adic String Theory

It has been conjectured that an extremum of the tachyon potential of a bosonic D-brane represents the vacuum without any D-brane, and that various tachyonic lump solutions represent D-branes of lower dimension. We show that the tree level effective action of p-adic string theory, the expression for which is known exactly, provides an explicit realisation of these conjectures.Comment: LaTeX file, epsf, 2 figures, 18 page

Standard Cosmology Delayed

The introduction of a delay in the Friedmann equation of cosmological evolution is shown to result in the very early universe undergoing the necessary accelerated expansion in the usual radiation (or matter) dominated phase. Occurring even without a violation of the strong energy condition, this expansion slows down naturally to go over to the decelerated phase, namely the standard Hubble expansion. This may obviate the need for a scalar field driven inflationary epoch.Comment: 12 pages, 3 figures, Revised with more elaborate discussions. Accepted for publication in JCA

Hybrid Inflation and Brane-Antibrane System

We study a string theory inspired model for hybrid inflation in the context of a brane-antibrane system partially compactified on a compact submanifold of (a caricature of) a Calabi-Yau manifold. The interbrane distance acts as the inflaton, whereas the end of the inflationary epoch is brought about by the rapid rolling of the tachyon. The number of e-foldings is sufficiently large and is controlled by the initial conditions. The slow roll parameters, however, are essentially determined by the geometry and have little parametric dependence. Primordial density fluctuations can be made consistent with current data at the cost of reducing the string scale.Comment: 22 pages, 7 Figs (added a Report-no and two references

Unbiased homomorphic system and its application in multiplicative noise reduction

Noise of a multiplicative nature is encountered in many applications such as coherent imaging, remote sensing and signal processing for communication systems. This thesis is concerned with the problem of reducing the multiplicative noise corrupting a signal. A generalization of the existing sampled function weighted order (SFWO) filter is proposed by relaxing the symmetry condition for the probability density function (PDF) of the noise to the filter. This generalized SFWO (GSFWO) filter is then used within a homomorphic system to reduce the multiplicative noise. It is shown that the output from such a system is biased, and hence, a suitable bias compensation technique is suggested. An unbiased homomorphic system, whose design is based on the PDF of the corrupting multiplicative noise, is proposed to reduce the noise. Images and videos generated by coherent imaging systems are always corrupted by speckle, which is a multiplicative noise having a lognormal distribution. A filter called the mean median (MM) filter to reduce additive white Gaussian noise (AWGN) is first proposed and this filter is then used within the unbiased homomorphic system to reduce the speckle in images. Fast filters to reduce AWGN in videos are also proposed in this thesis. Novel techniques for temporal estimation employing a change detection technique to determine the interframe motion are developed for their use in these filters. A new method is proposed to appropriately combine the spatial and spatiotemporal estimates of the original signal in order to obtain the final output. Finally, the MM-filter along with a novel temporal estimation scheme are used within the unbiased homomorphic system to reduce the speckle in videos. The effectiveness of the various proposed algorithms is demonstrated and compared with that of some of the existing schemes through extensive simulation